Method for obtaining electronic color correction signals

ABSTRACT

A method for obtaining electronic color correction signals in which the differences with respect to compensative masking (elimination of the gray tones) are formed between the amplitudes of the logarithmic or trichromatic color measurement signals nonlinearly transformed. These difference signals are split by selection according to their sign into primary partial signals from each two of which a further signal may be formed and divided into secondary partial signals according to actual sign. An emphasis of primary and/or secondary partial signals is provided wherein the signals are emphasized more strongly in definite areas of the color space than in other color areas of the region of the color space encompassed by the partial signals. Relative emphasis is effected only over a relatively limited area within the color scale extending from a neutral (non-chomatic) color to a saturated chromatic color selectively by transforming the partial signals in accordance with a non-linear function. The same partial signal may be transformed in accordance with two different functions, for example, a logarithmic and a negative linear function, and the difference signal is formed from the two resulting transformation signals. The signals are then employed to control color amplifying channels in the color correction process.

United States Patent 11 1 Keller et al. 1

METHOD FOR OBTAINING ELECTRONIC Sept. 11,1973

ABSTRACT A method for obtaining electronic color correction signals inwhich the differences with respect to compensative masking (eliminationof the gray tones) are formed between the amplitudes of the logarithmicor trichromatic color measurement signals non-linearly transformed.These difference signals are split by selection according to their signinto primary partial signals from each two of which a further signal maybe formed and.

' nal may be transformed in accordance with two different functions, forexample, a logarithmic and a negative linear function, and thedifference signal is formed from the two resulting transformationsignals. The signals are then employed to control color amplifying COLORCORRECTION SIGNALS [76] inventors: Hans 161E 23 Kiel, FlensburgerStrasse 23'; Hans-Georg Knop, Am 2323 Asch berg; Horn22,

Germany [221' fined; f1if25fffi 211 Appl. No.: 212,182

Related US. Application Data [63] Continuation-impart of Ser. No.847,970, Aug. 6,

1969, abandoned.

[30] Foreign Application Priority Data v Aug. 8, 1968 Germany P 17 97049.9

52 US. Cl 178/5.2 A [51] Int. Cl. H04n 1/46 [58] Field of Search l78/5.2A

[56] References Cited UNITED STATES PATENTS 3,600,505 8/l971 Dobouney178/52 A 2,949,499 8/l960 Zeyen et al. 178/52 A 3,324,235 6/1967 Kytel78/5.2 A

Primary Examiner-Robert L. Griffin Assistant ExaminerGeorge G. StellarAYibHEy Carlton Hill, J. Arthur Gross and John D. Simpson et al.

channels in the color correction process.

14 Claims, 11 Drawing Figures Patented Sept. 11,1973 3,758,707

6 Shanta-Shut 5 45' 46' 47' 46 49 63 I13 114" 115119 720 +100% pos negpos neg pos neg pos pos I while V 100 l.

magenla brown A Fly. 77

STAGE I N VENT 0R5 Hans Keller Hans-Georg Knop 13% M ATTORNEYS PatentedSept. 11, 1973 whiie signal ampl.

brown 400 B 5006 600R 700,um

6 Shoots-Shut 6 Fig. 70

INVENTORS Hans Keller Hans-Georg Knop Lax w aa/ ATTORNEYS METHOD FOROBTAINING ELECTRONIC COLOR CORRECTION SIGNALS CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-in-part of our previousapplication of the same title, Ser. No. 847,970, filed Aug. 6, 1969, nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method for obtaining color correction signals, and moreparticularly to the realization of color correction signals which areeffective to provided fidelity in the reproduction of skin colors, brownand several light pure colors.

2. Description of the Prior Art In color printing most color correctioncomputers, so-called color scanners, color correction techniques work onthe basis of trichromatic color measurement values obtained throughcolor filters, which measurement values, in the space of "Cartesiancoordinates form the group of all the colors, a color space of irregularshape. Associated therewith a cubic color spaceof the color dosagevalues, utilized for color printing. The mathematical combination whichconverts the space of the trichromatic color measurement values intothat of a color dosage value is the color correction calculation. Thefirst step is usually a non-linear distortion, generally meaning alogarithmic or a part-logarithmic evaluation of the color measurementvalues according to said distortion and the color space is shown as asmall irregular rhombus which has surfaces of different inclination,curvature and size. Correction signals, then, are derived from thetransformed trichromatic signalsto which the correction signals are thenagain added, addition and subtraction of signals having the character ofa linear transformation of the color space. Thereby,

however, residual errors remain which have to'be elimi-- nated byaltering the method of calculation or by additional calculation. Thecorrection signals are effective in accordance with a previous methodover a large area of the color space.

. In accordance with modern methods, colorcorrec-i tion signals becomemore and more specific in that they are required to be effective only onone part of the color space. It is knownto obtain the difference fromtwo trichromatic signals, in which difference no signal for a neutraltone is contained. This signal has already been submitted to lineardistortion. It is further known to split this primary difference signalaccording to sign into a positive and negative partial signal and to useknown also to submit such a signal for the purpose of contrastadjustment to a simple non-linear transformation, before it comes intouse as a correctionsignal. By

utilizing such correction values, calculation of the ideal color dosagespace can be carried out successfully from the color measurement valuespace. In spite of the above techniques, however, there are stilldefects. The color correction values always have a very monotonouslyconstant, mostly linear curve in the area of operation. The signals areoriented therein with reference to the usual control method, in order tocorrect preferably control colors, more particularly the six chromaticdial colors (3 print colors and their 3 mixtures of the first order) andthe gray tone. The correction of the intermediate colors of the colorspace is no less important, sometimes it is even especially important.This concerns particularly the skin colors" brown and several light purecolors.

SUMMARY OF THE INVENTION According to the invention, techniques areprovided to obtain correction signals for special correction ofintermediate colors and according thereto, the primary and/or thesecondary partial signals are submitted before their furtherutilization-to an emphasis, the signals being emphasized more stronglyin definite areas of the color space than in other color areasof theregion of the color spacing encompassed by the partial signals.

Since the-relative emphasis is to be effected only over a relativelylimited area within the color scale extending from a neutral(non-chromatic) color to a saturated chromatic color, that is to sayselectively in these ranges, the emphasis is effected advantageously bytransforming the partial signals in accordance with a non-linearfunction.

The possible achievement of a definite maximum color correctioncomprises, in particular, the provision of a transformation of the samepartial signal with two different functions, preferably a logarithmicand a negative linear function, and the difference is formed from thetwo resulting transformation signals.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention, its organization, construction and operation will bestbe understood'by reference to the following detailed'description, takenin conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 together form a schematic circuit diagram of an embodimentof a color scanner'for color correction over several electronic stagesconnected between the points of scanning and recording;

FIG. 3 is a schematic circuit diagram of a simplified color correctioncircuit with respect to that illustrated in FIGS. 1 and 2;

FIG. 4 is a schematic circuit diagram of a logarithmic amplifier;

FIG. 5 is a schematic circuit diagram of an inverter circuit; I

FIG. 6 is a schematic circuit diagram of an addition and separationcircuit for adding and then separating the positive and negativeportions of the sum signal;

FIG. 7 is a schematic diagram of a sign inverter;

FIG. 8 is a schematic circuit diagram of a non-linear amplifier;

FIG. 9 is a schematic circuit diagram of a correcting stage which may beutilized in practicing the present invention;

FIG. 10 is a graphical illustration of the amplitude path of thelogarithmic cover'separation signals; and

FIG. 11 is a schematic representation of the output signals of theindividual stages of FIGS. 1, 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The color components containedin the actual color measurement value signals are referenced herein andon the drawings by lower case letters where wh means white, bl black, rdred, gn green, ye yellow, cy cyan, mg magenta and vi violet.

In FIGS. 1 and 2, a plurality of rotating drums 1, 2, 3, 4 and 5 aresynchronously driven by a motor 6 via a schematically illustrated shaft7. The dmm 1 carries a colored image pattern 8 which is electroopticallyscanned and reproduced as color separation 9, 10, 11 and 12 on therecording drums 2, 3, 4 and 5, respectively. When the image pattern 8 isscanned, the passed or reflected light of a light beam 13 is focussed bymeans of lenses 14 and 15 and passed on through an, aperture 16. Thefocussed light beam 13 is trichromatically split by means ofinterference filters 17, 18 and 19, or by means of correspondinglycolored glass filters, to become three light beams 21, 22 and 23. Thetrichromatically split beams 21, 22 and 23 are received by respectivephoto tubes 24, 25 and 26 to develop the corresponding color separationsignals which are adjustable by means of potentiometers 27, 28 and 29 inorder to supply respective preamplifiers 30, 31 and 32 with equal inputvoltage levels. Gamma correction stages 33, 34 and 35, linearlogarithmic distortion amplifiers 36, 37 and 38 and distortion stages39, 40 and 41 are serially provided for each of the pre-amplifiers 30,31 and 32. The uncorrected primary color separation signals at theoutputs of the logarithmic amplifiers 36, 37 and 38 are proportional tothe density of the colored image patterns and are sumbitted to a firstcolor correction. For this purpose, the primary color separastages 48,49 and 50. The lines 42-44 represent different channels.

The addition and separation stage 48 includes an adding circuitcomprising a pair of resistors 481 and 482 and a rectifier circuitincluding a pair of diodes 483 and 484. The adding and separation stages49 and 50 contain similar structure and have corresponding referencenumerals. A positive primary color separation signal of one color and anegative, i.e., inverter signal, of another color are added each time inthe addition and separation stages 48, 49 and 50 in order to obtainfirst difference signals. These first difference signals are divided,according to their sign, into two correction signals. Each one of thesecorrection signals represents only two dial colors (basic colors). Thewhite and gray portions are completely removed. These differencesignals, i.e., the first color correction signals are extended towardthe uncorrected color separation signals for carrying out a firstcorrection by way of lines 51, 52,.

53, 54, and 56 and respective adjustable resistors 57, 58, 59, 60, 61and 62. The strength of this correction can be adjusted independentlyfor each correction signal and for each polarity by the resistors 57-62.In order to be able to carry out a second and more particular colorcorrection, pairs of the first difference sig- 4 way that they havesignal components which represent the same dial (basic) colors mentionedabove, but with different signs. These selected signals are applied to asecond group of addition and separation stages as 63,

64, 65, 66, 67 and 68, which are the same in structure as the stages48-50 but including adjustable resistors,

and in which the amplitude-relationship can be 'adjusted by theadjustable resistors sothat the signals, although appearing withdifierent polarities, will have the same amplitude. Due to the oppositepolarity of the signal components, one dial (basic) color of the twosig-,

The second color correction process for only one pri-:

mary color separation signal for the separation signal of the redfilter, cyan cy, is illustrated in FIGS. 1 and 2. For the second colorcorrection, pairs of the same dial (basic) color, but with differentsigns, are selected and applied to the first corrected signals by way ofthe potentiometers 69-74 and the resistors -80, for example, via a line81. The amount of correction can be adjusted by way of thepotentiometers 69-74 to allow cor rection of each one of the six dial(basic) colors.

The resistors 82-84 are provided for decoupling the two color correctionstages.

After the secondcolor correction, the twice corrected signals areapplied to a plurality of amplifiers 85, 86 and87, and processed towarda black-selecting or separation stage 88 for selecting the maximumsignal. This maximum signal among the color separation signals isrepresented by the black separation signal which is provided to a colorreturning or under-color removal stage 98 from where it is applied to agradation stage 100 by way of an amplifier 99. The black separationsignal is applied tothe corrected color separation signals viaadjustable resistors 981, 982 and 983 in order to reduce the contents ofthe colors in the black 'and gray tones. The resulting color separationsignals are non-linearly distorted or reshaped in the gradation stages95, 96, 97 and 100 in order to adapt the signals to the particularitiesof the recording system, i.e.,- the particularities of the printedshapes and the photo material.

The amplitudes of the reshaped signals are adjusted according to thedesired intensity values by way of respective potentiometers 101, 102,103 and 104. The recording lamps 105, 106, 107 and 108 are controlled bythese signals and produce the desired color separations on the photosensitive photo material 9, 10,11 and 12 by way of corresponding opticalfocussing systems 109, 110, 111 and 112.

In order to obtain a correction possiblity for the socalled skin colors,such as brown, light red and some lighter or fairly light colors, whichshould be independent from the above described correction of the sixdial (basic) colors of the color range, the positive partial signalcorresponding to red rd is taken from an output of the addition andseparation stage 63 (FIG. 1) for application linearly through the valuesfrom zero through saturation in a linear stage 113. The emphasis of thissignal, which is suggested according to this invention, is obtained bymeans of non-linear distortion.

For this purpose, a superposition of a linear function and a non-linearfunction, which by itself is not suitable, is effected which results ina non-linear distortion. The positive linear connection between signalamplitude and color correction degree is inverted in the stage 113,i.e., is rendered negatively linear. The signal which is supplied to thestage 113 is simultaneously applied to a stage 114 in which itexperiences a logarithmic distortion. A summation signal is obtainedfrom the output signals of the stages 113 and 114 in a stage 115 by wayof resistors 116 and 117 and, due to the negative sign of the outputsignal of the stage 113, the summation signal represents the differenceof the two signals. If, in addition, precautions are taken for renderingthe output voltages of the stages 113 and 114 for the saturated colorsequally large, the curved path 118 occurs as illustrated in the stage115, i.e., a curve which has the value zero with white wh or black bland with saturated red rd and which has a maximum for certainintermediate colors. By means of a selection of the applied conversionpossibilities or, respectively, their coefficients and constants, it canbe provided that this maximum lies about a particularly selectedintermediate color.

The above described circuit, however,- does not operate as unambiguouslyas desired sincethe intermediate values for the colors white wh throughred rd are light red colors and the intermediate values from black blthrough red rd represent brown tones. In order to select only the lightred, the brown color may be corrected, or both colors can be correctedto differentdegrees. The output signal of the stage 115 issimultaneously applied to two different amplifier stages 1l9and 120.Only one of these amplifier stages is open when white wh colors occurand only the other stage opens when primarily black bl colors occur. Theamplifiers 119 and 120 are realized by two double grid tubes 121 and 122and one of their control grids 123 and 124, respectively, is loaded withthe output signal of the stage 115.

The second grid 125 of the tube 121 is loaded by way of a line 126 witha positive white +wh voltage occur ring at the output of the stage 47'(FIG. 1). The tube 121 is therefor only controlled when fairly lightcolors occur, i.e., when high voltages are present at its grid 125, andit remains blocked for black bl colors. Therefore, only signals forlight red colors, in particular flesh colors, will occur at the output127 of the tube 121 as indicated graphically by the curve 128 adjacentthe amplifier 119. i

The second control grid 129 of the tube 122 is loaded with a negativewhite -wh signal from the stage 47 by way of a resistor 130 and a line131 and is positively based by way of a resistor 132. The bias iscompensated by high negative white -wh signals or even over cornpensatedso that the tube 122 is blocked for colors close to white wh. However,the tube 122is controlled or treated into conduction with colors closeto black bl,'

since the positive bias prevails toward such operation. Therefore,particular signals for the brown colors occur at the output 133 of thetube 122, as can be seen from the curve 134 by graphically illustratedadjacent the amplifier 120. The voltage at the output 133 of the tube122 of the amplifier 120, as well as the voltage at the output of thetube 121 of the amplifier 119 is applied the twice corrected colorsaccording to, for example, flesh tones.

However, on the other hand, it can also happen that a saturated color issupposed to be corrected without permitting the correction to have anefi'ect on the intermediate colors. For this reason, a furthercorrecting stage 141 (FIG. 1) is provided to obtain a signal having thenegative yellow -ye and negative green -gn from the stage 49 by way ofan inverter 142. The original linear voltage curve black bl to yellow yeis distorted according to the path of a curve with increasing slope asillustrated in the correction stage 141. This distortion is obtained inone or several diodes which are operated in the bent portion of theircharacteristic curve. One of the two signal components minus green -gnand minus yellow -ye, the component minus green -gn representing anintermediate value and thus being essentially smaller than the componentminus yellow -ye, is more or less completely suppressed, depending onthe position and shape of the curve, or rather the component minusyellow ye is strongly emphasized.

Therefore, a particular correction signal for the saturated yellow yewill appear at the output 143 of the stage 141 and applied to the mainchannels (lines 42, 43 and 44) by way of adjustable resistors 144, 145and In FIG. 3, another embodiment of the invention has been illustratedin which the correction of the bright red colors, the brown colors andthe saturated yellow is effected with, in principle, the same correctionstages as in FIGS." 1 and 2. However, the individual stages which wererequired with the preceding example for the separation and correction ofthe six colors, have been eliminated. Furthermore, in order to simplifymatters, the scanning drum, thefbeam separation apparatus, the leveladaptation apparatus and the gamma correction apparatus for obtainingthe uncorrected primary color separation signals have been omitted onthe scanning side. On the recording side, the black separation stage,the color return or removal circuits, the gradation control and therecording circuits and recording drums have not been illustratedalthough these equipments are also utilized with the apparatusspecifically illustrated in FIG. 3.

The electrical color separation signals which are obtained behind theseparation filters arrive at the termirials R, G and B. The terminals R,G and B have also been indicated in FIG. 1 in order to point out wherethe electrical color separation signals are correspondingly provided.The color separation signals are then logarithmatized in the mannerillustrated in FIG. 1 in the stages 39, 40 and 41, or convertedaccording to a similar non-linear function such as a so-calledsemilogarithmatizing function, and are separated according totheir-sign, or with alternating voltages according to their phaseposition, in the stages 45', 46' and 47 by the respective inverters45,46 and 47, and are further processed on separate channels. The furtherapplication of these signals has been illustrated only for one of-thes'e c hann els for-purpose of simplicity and clarity.

The color portions which are contained in the respective signals arerecorded at the outputs of the stages. In the stages'48 and 49 two ofthe signals, respectively with opposite signs are added by way of theresistors 48.1, 482 or491 and 492 and separated according to their signsby means of the diodes 483 and 484 or 493 and494. These signals are thefirst difference signals and respectively are contained only to remnantcolor components. In FIG. 1 these have been illustrated as positivemagenta +mg, positive red +rd, and negative cyan cy, negative green gn,and negative magenta mg, negative violet vi and positive yellow +ye,positive green i-gn. in the stage 63, the signal containing the negativemagenta and violet componets mg, vi, which occurs at the output of thestage 49 is added to the signal containing the positive magenta andpositive red +mg, +rd which is provided by the stage 48, by way of theresistors 631 and 632. Due to the opposite signs of the positive andnegative magenta color portions +mg, mg, the magenta color portion iseliminated and, after the sign separation provided by the diodes 633 and634,- the signals of positive red +rd and negative violet vi areobtained as the second difference signals or secondary partial signals.In the example according to FIG. 3, only the further treatment of thepositive red +rd difference signal is specifically illustrated.

As it has been indicated above, the positive red +rd difference signalcan be extended through to the saturated color, whereby the signalvoltage starting from the value zero increases linearly toward the valueof saturation.

The emphasis which is suggested according to the present invention isnow obtained by a non-linear distortion of this linear relationship.

A suitable non-linear distortion can also be provided by means ofsuperimposing a linear function with a non-linear function which, asmentioned above,-is not suitable by itself. This is accomplished in thestages 113, 114 and 115. In the stage 113, the positive linearrelationship between signal amplitude and color saturation degree isinverted, i.e., rendered negatively linear, as indicated by thegraphical representation 113'.

The positive red +rd signal which is applied to the stage 113 issimultaneously applied to the stage 114 where it experiences alogarithmic distortion, as graphically indicated at 1 14'. The outputsignals of the stages 1 l3 and 114 are added in the stage 115, i.e., thedifference is formed over the resistors 116 and 117 due to the negativesignal of the stage 113. If, in addition, provisions are made that theseoutput signals are equally large for the saturated color, and thusresults in the difference of zero, the voltage curve 118 will result,namely a curve which has the values zero with white wh (or black bl) andwith saturated red rd, and which has a maximum for certain intermediatecolor tones. By means of a suitable selection of the conversionfunctions, or rather their coefficients and constants, it can beprovided that this maximum lies at the particular desired intermediatevalues.

The circuit described thus far, however, still operates am biguouslysince the intermediate values for the color line white wh through red rdare light red colors and the intermediate values for the color lineblack bl through red rd are brown colors. In order to obtain thatselectively only the light red rd or the brown colors or both colors canbe corrected to different degrees, the output signal of stage 115 issimultaneously applied to two different amplifier channels, one of whichis opened only when colors appear which are close to white wit and theother only when colors appear which are close to black bl.

The amplifier channels 119 and 120 are realized by the two double gridtubes 121 and 122, whereby the first control grid 123 of the tube 121 isloaded with the output voltage of the stage 115. The second grid 125 ofthe tube 121 is loaded by way of the line 126 with a positive whitesignal +wh which occurs at the output of the stage-47. This tube istherefore only triggered when light colors occur, when high positivevoltage is applied to its second control grid 125, while it remainsblocked for signals close to black bl. Therefore, only signals for lightred rd colors, in particular flesh colors, can occur at the output 127ofthe tube 121, as is indicated graphically by the curve 128.-Thesesignals are applied toward the main channels 42, 43 and 44 by way of theresistors 138, 139 and for carrying out the correction as in FIG. 2. Thefirst control grid 124 of the tube 122 is loaded with the output voltageof the stage 115. The second control grid 129 of the tube 122 is loadedby way of the line 131 and a resistor 130 with a negative white whsignal, and is positively biased by way of the resistor 132. The bias iscompensated or even over-compensated by high negative white wh signalsso that the tube 122 is blocked for colors close to white wh. Withcolors close to black bl, however, the tube 122 is triggered intoconduction due to the prevailing positive bias. Therefore, particularsignals for brown colors occur at the output 133 of the tube 122 as canbe seen from the graphical illustration at.l34. The correction iseffected as in FIG. 2 via the resistors 135, 136 and 137 on the mainchannels 42, 43 and 44.

On the other hand, it can also happen that a saturated color is supposeto be corrected, but without having an effect on the intermediate colorvalues. Such a correc tion signal is obtained in the stage 141 from theprimary positive yellow and green +ye, +grr signal which is taken fromthe addition and separation stage 49. In the:

correction stage 141, the originally linear voltage curve blackblthrough yellow ye is distorted according to the curve 141 so as to haveincreasing slope. This distortion is effected in a manner which iscommon through the use of one or several diodes which operate in thebent portions of their characteristic curves. The positive green +gncolor portion of the two signal portions, which is an intermediate valueand thus essentially smaller than the positive yellow +ye portion isstrongly emphasized with respect to the yellowye. Therefore, a specialcorrecting signal for the saturated yellow ye appears at the output 143of the stage 141 and applied toward the main channels 42, 43 and 44 byway of the adjustable resistors 14-4, 145 and 146."

FIG. 4 illustrates a circuit of a logarithmic amplifier as may beutilized for the stages 39, 40 and 41 when the circuit construction isto be realized for direct voltage signals. The input of the amplifierreaches a first input; 241 of an operational amplifier 242 by way of aresistor- 240 and the second input 243 of the operational amplifier 242is grounded. The input 241, a plurality of resistors 244-247 and aplurality of diodes 248-250 are connected to a voltage divider circuitcomprising a plurality of resistors 251-254 which is connected between apositive pole of a voltage source and the resistor 244. A resistor 255is connected to the output ofthe amplifier 242 and to the resistor 244.By means of selecting the resistors to determine the operationalpointsofthe diodes 248 249 and 250, the curve path of a logarithmic function eanbe approximated according to whose characteristic the logarithmicamplifiers are to operate. In order to obtain more accuracy, the numberof diodes and the corresponding resistors can be increased.

If the signals which are to be processed are not to be direct voltagesignals but direct voltage signals which are modulated onto an alternatefrequency carrier, a circuit can advantageously be provided such asdescribed in U.S. Pat. application Ser. No. 99,011, filed Dec. 17, 1970in FIG. 5. Y

FIG. illustrates a. circuit according to which the stages 45', 46 and 47may be constructed. This circuit is an inverter circuit which, at itsoutput, provides a signal having a positive sign and the same signalhaving a negative sign. The input signal at the input 256 is connecteddirectly to an output 258 without sign inversion by way of a line 257,and by way of a resistor 259, an input 260 of an operational amplifier261 whose other input 262 is grounded. The input 260 is connected withthe output 264 by way of a resistor 263 and a signal will be provided atthe output 264 whose sign is opposite to that of the input signal andtherefore opposite to that of the output signal at the output 258. Thiscircuit is suitable for direct voltage signals. However, if alternatevoltage signals are to be utilized, in the form of signals which aremodulated onto a carrier, a circuit may be applied as it is illustratedin FIG. 9 of the aforementioned U. S. Pat. application 99,011, filedDec. 17,

FIG. 6 illustrates a further circuit which permits realization of thestages 48, 49 and 50, as well as the stage 63. The inputs 265 and 266are connected to a resistor 269 by way of respective resistors 267 and268. The resistor 269 is connected to an input 270 of anoperationalamplifier. The resistors 267 arid 268 correspond to the resistors 481and 482, 491 and 492, 501 and 502, and 631 and 632 of FIGS. 1, 2 and 3.The other input 272 of the operational amplifier 271 is connected toground. Two diodes 275 and 276 connect the output of the amplifier 271to respective outputs 277 and 278, the diodes 275 and 276 beingconnected in an antiparallel relationship. The input 270 of theamplifier 271 is coupled back from the outputs 277 and 278 by way ofrespectiveresistors 279 and 279'. A similar circuit for adding andseparating direct current signals is illustrated in FIG. 60f theaforementioned U. S. Pat. application Ser. No. 99,011, filed Dec. 17,1970. A corresponding circuit for the case wherein alternate voltagesignals are utilized can also be applied in an advantageous manner asillustrated in FIG. 7 of this prior application.

FIG. 7 illustrates a sign inverter which comprises an operationalamplifier circuit including an amplifier 261 having an input 262connected to ground and an input 260 connected to an input terminal 256by way of a resistor 259. The input 260 is also connected to the outputof the amplifier 261 by way of a resistor 263, an output terminal forthe circuit being provided by means of the terminal 264. This invertercircuit may be utilized for realizing the sign inverter circuits such as45, 46, 47.

FIG. 8 illustrates a circuit similar to FIG. 4 which may be employed inrealizing the stage 114. The signal which is applied to the input 280reaches the input 282 of anoperational amplifier 283 by way of aresistor 281. The other input 284 of the operational amplifier 283 isconnected to ground. A resistor 286 connects the output 285 to the input282 by way of a resistor 287. A voltage divider comprising a pluralityof resistors 288, 289 and 290 is connected from the connection point ofthe resistors 286 and 287 to a positive pole of a voltage source. Theconnection points between'the resistors 288 and 289 as well as betweenthe resistors 289 and 290 are respectively connected with the input 282of the amplifier 283 via a series connection of a diode 291 and aresistor 292, and a diode 293 anda resistor 294, respectively. Dependingon the selection of the resistors of this network, the desired functionscan be approximated by the diodes.

' In FIG. 9, a circuit has'been illustrated for performing the function141 of the stage 141 of FIGS. 1 and 3. By way of a plurality ofresistors 296, 297 and 298, an input 295 is connected to a voltagedivider comprising a plurality of resistors 299, 300, 301 and 302 whichare connected between ground and a constant negative voltage. Thevoltage divider is connected with an input 306 of an operationalamplifier 307 by way of a plurality of diodes 303, 304 and 305. Theother input 308 of the operational amplifier 307 is connected to ground.The output 309 of the operational amplifier 307 is connected to theinput 306 by way of a resistor 310. The diodes 303-305 are blocked inresponse to low signals due to the negative voltage applied by way ofthe voltage divider. The more positive the input signal becomes, themore will bepassed by the diodes. The passage characteristic of thisnetwork corresponds to the passage curve of the stage 141 in FIGS. 1 and3, that is by the curve 141. g

In order to clearly express with this invention which colors or signalamplitudes in the spectrum ranges, respectively, are meant by the colorterms applied herein such as white wh, magenta mg, red rd, light red orbrown, the amplitude path of the logarithmized signals of these colorsis illustrated in F I6. 10, while depending on the wave lengths 400 umthrough 700 um. InFIG. 10 the letters R, Gand B state the spectrumranges of the red, blue and green filters. From the above, it canclearly be seen that, a side of the filter ranges characteristic to'theindividual colors, signal amplitudes occur in other filter ranges withthese individual colors which must not be neglected and which,.amongother things, are the reason for the required color corrections.

The signal amplitude for white wh is adjusted for all channels topercent with such a color correction device which results in a clearassignment of all signal magnitudes which are obtained for the colors.The output signals for the colors occurring have schematically beenillustrated as arrows in FIG. 11 for the individual stages of thecircuit arrangement of FIGS. 1, 2 and 3, which arrows are respectivelyseparated for the positive and negative polarities. The referencenumerals 45', 46 ,47, 48, 49, 63, 113, 114, 115, 119 and 120 in theupper line of FIG. 11 indicate the outputs of the likenumbered stages ofFIGS. 1, 2 and 3. The signal arrows under these reference numeralsindicate the signal magnitude for the respective color pattern.Therefore, it can be recognized that the signal white wh at the positiveoutput of the stage 48 has already been compensated to the value zero,whiel the signals for magenta mg and red rd occur fully since they arenot present as negative signals and the output of the stage 46. Acorrespondingly smaller signal of the stage 48 occurs for the colorlight red from the addition of the positive output of the stage 45' tothe negative output of the stage 46. The signal representing brown alsooccurs in the, stage 48 with about half the amplitude. If the signal oflight red and brown remain constant. In the stage 114, the portions oflight red and brown are increased relatively with respect to the signalfor red rd, but not in the stage 113 light red and brown remain constantin the stage 115, while red rd is compensated to zero. The remainingpartial signals can now be amplified and, as described above, be appliedto the main channels 42, 43 and 44 of FIGS. 1 and 2 for a correction byway of the amplifiers 119 and 120.

Although we have described our invention by reference to specificillustrations, many changes and modifications of our invention maybecome apparent to those skilled in the art without departing from thespirit and scope of our invention. It is therefore to be understood thatwe intend to include within the patent warranted hereon all such changesand modifications as may reasonably and properly be included within thescope of our contribution to the art.

What we claim is:

1. In a method for obtaining electronic color correction signalsincluding generating color measurement signals, transforming the colormeasurement signals in a non-linear fashion and dividing the transformedsignals according to sign, combining the divided signals and dividingagain according to sign into primary partial signals, and combining theprimary partial signals and dividing according to sign into secondarypartial signals, theimprovement in the method comprising furthertransforming the partial secondary signals by selectively emphasizingthose signals in definite areas of color space more strongly than thosesignals in other areas of the color space encompassed by the secondarypartial signals in accordance with a transfer function which isemphasized between a nonchromatic color and a chromatic color.

2. In the method according to claim 1, wherein said furthertransformation of the partial secondary signals is further defined ascomprising the step of emphasizing said signalswith the emphasis lyingat a saturated color in accordance with a selected non-linear function.

3. In a method according to claim 2, wherein the further transformationof the partial secondary signals comprises non-linearly emphasizingsignals lying in the intermediate values of the color scales extendingfrom white to a saturated color.

4. In a method according to claim 2, wherein the further transformationof the partial secondary signals comprises non-linearly emphasizingsignals lying in the intermediate values of the color scales extendingfrom black to a saturated color.

5. In a method according to claim 1, wherein the fur ther transformationof the partial secondary signals comprises the step of emphasizing thepartial signals in accordance with a predetermined non-linear function.

6. In a method according to claim 5, wherein the nonlinear emphasis isprovided with the emphasis lying in a saturated color.

7. In a method according to claim 1, wherein the further transformationof the partial secondary signals includes transforming a partial signalin accordance with a logarithmic function, transforming the same partialsignal in accordance with a negatively linear function,

r and providing the difference between'the two transformed signals.

8. In a method according to claim 7, wherein the further transformationof the partial secondary signals includes providing that the differencebetween the two transformed signals is zero for predetermined colors.

9. In a method according to claim 8, wherein the fun ther transformationof the partial secondary signals further includes feeding the differencesignal to two differ ent channels which correspond to the terminals ofthe color space area encompassed by the secondary partial,

signals, and controlling the opening and closing of the channels inaccordance with the magnitude of the difference.

, 10. In a method according to claim'9, comprising the further steps ofderiving an additional primary partial signal from a color measurementsignal and providing 7 the additional partial signal to each of thechannels to aid in the opening and closing thereof.

11. In a method according to claim 7, wherein the further transformationof the partial secondary signals includes emphasizing those signalswhich lie intermediate the ends of the color space area encompassed by Isignals, and controlling the opening and closing of the channels inaccordance with the magnitude of the difference.

14. In a method according to claim l3,"comprising the further step ofproviding an additional partial signal to each of the channels to aid inthe opening and closing thereof.

* t l t

1. In a method for obtaining electronic color correction signalsincluding generating color measurement signals, transforming the colormeasurement signals in a non-linear fashion and dividing the transformedsignals according to sign, combining the divided signals and dividingagain according to sign into primary partial signals, and combining theprimary partial signals and dividing according to sign into secondarypartial signals, the improvement in the method comprising furthertransforming the partial secondary signals by selectively emphasizingthose signals in definite areas of color space more strongly than thosesignals in other areas of the color space encompassed by the secondarypartial signals in accordance with a transfer function which isemphasized between a nonchromatic color and a chromatic color.
 2. In themethod according to claim 1, wherein said further transformation of thepartial secondary signals is further defined as comprising the step ofemphasizing said signals with the emphasis lying at a saturated color inaccordance with a selected non-linear function.
 3. In a method accordingto claim 2, wherein the further transformation of the partial secondarysignals comprises non-linearly emphasizing signals lying in theintermediate values of the color scales extending from white to asaturated color.
 4. In a method according to claim 2, wherein thefurther transformation of the partial secondary signals comprisesnon-linearly emphasizing Signals lying in the intermediate values of thecolor scales extending from black to a saturated color.
 5. In a methodaccording to claim 1, wherein the further transformation of the partialsecondary signals comprises the step of emphasizing the partial signalsin accordance with a predetermined non-linear function.
 6. In a methodaccording to claim 5, wherein the non-linear emphasis is provided withthe emphasis lying in a saturated color.
 7. In a method according toclaim 1, wherein the further transformation of the partial secondarysignals includes transforming a partial signal in accordance with alogarithmic function, transforming the same partial signal in accordancewith a negatively linear function, and providing the difference betweenthe two transformed signals.
 8. In a method according to claim 7,wherein the further transformation of the partial secondary signalsincludes providing that the difference between the two transformedsignals is zero for predetermined colors.
 9. In a method according toclaim 8, wherein the further transformation of the partial secondarysignals further includes feeding the difference signal to two differentchannels which correspond to the terminals of the color space areaencompassed by the secondary partial signals, and controlling theopening and closing of the channels in accordance with the magnitude ofthe difference.
 10. In a method according to claim 9, comprising thefurther steps of deriving an additional primary partial signal from acolor measurement signal and providing the additional partial signal toeach of the channels to aid in the opening and closing thereof.
 11. In amethod according to claim 7, wherein the further transformation of thepartial secondary signals includes emphasizing those signals which lieintermediate the ends of the color space area encompassed by thesecondary partial signals.
 12. In a method according to claim 11,wherein the further transformation of the partial secondary signalsincludes providing that the difference between the two transformedsignals is zero for predetermined colors.
 13. In a method according toclaim 11, where the further transformation of the partial secondarysignals further includes feeding the difference signal to two differentchannels which correspond to the terminals of the color space areaencompassed by the secondary partial signals, and controlling theopening and closing of the channels in accordance with the magnitude ofthe difference.
 14. In a method according to claim 13, comprising thefurther step of providing an additional partial signal to each of thechannels to aid in the opening and closing thereof.